An organic electroluminescence device, which includes an organic emitting layer between an anode and a cathode, has been known to emit light using exciton energy generated by a recombination of holes and electrons that have been injected into the organic emitting layer.
Such an organic electroluminescence device, which has the advantages as a self-emitting device, is expected to serve as an emitting device excellent in luminous efficiency, image quality, power consumption and thin design.
For usage of an emitting layer in the organic electroluminescence device, a doping method, according to which an emitting material (dopant material) is doped to a host material, has been known as a usable method.
In this method, in order to efficiently generate excitons from the injected holes and electrons and efficiently convert the generated exciton energy into emission, exciton energy generated in the host is transferred to the dopant and the dopant emits light.
A typical emitting material used for an organic electroluminescence device is a fluorescent material that emits fluorescent light by a singlet exciton. However, recently, it is suggested to use a phosphorescent material that emits phosphorescent light by a triplet exciton instead of the fluorescent material (see, for instance, non-Patent Documents 1 and 2).
Since the internal quantum efficiency can be enhanced up to approximately 100% in theory by using such a phosphorescent material, an organic electroluminescence device having high efficiency and consuming less power can be obtained.
In order to intermolecularly transfer the energy from a phosphorescent host to a phosphorescent dopant (phosphorescent material) in a phosphorescent-emitting layer formed by the phosphorescent material being doped, triplet energy gap Eg(T) of the phosphorescent host is required to be larger than triplet energy gap Eg(T) of the phosphorescent dopant.
CBP is typically known as a material having an effectively large Eg(T).
By using CBP as the phosphorescent host, energy can be transferred from the phosphorescent host to the phosphorescent dopant exhibiting a predetermined emission wavelength (e.g., green, red), by which a phosphorescent-emitting device of high efficiency can be obtained (see, for instance, Patent Document 1).
However, although an organic electroluminescence device in which CBP is used as the host material exhibits much higher luminous efficiency due to phosphorescent emission, the organic electroluminescence device has such a short emission lifetime as to be practically inapplicable.
On the other hand, a variety of host materials for fluorescent dopants are known. Various proposals have been made on a fluorescent host, with a combination of a fluorescent dopant, excellent in luminous efficiency and emission lifetime.
However, although the fluorescent host has larger singlet energy Eg(S) than the fluorescent dopant, the fluorescent host does not necessarily have a large Eg(T). Accordingly, it is not successful to simply apply the fluorescent host to the phosphorescent material.
Well-known examples of such a fluorescent host are an anthracene derivative, pyrene derivative, naphthacene derivative and the like. However, for instance, the anthracene derivative has Eg(T) of approximately 1.9 eV, which is insufficient for emitting light of a wavelength in a visible light range of 450 nm to 750 nm. Accordingly, the anthracene derivative is not suitable as the phosphorescent host.
When electrons injected into the emitting layer are insufficient in volume, holes injected from the anode to the emitting layer is not recombined with electrons in the emitting layer and are possibly transferred into the cathode.
In order to solve such a problem, there has been known a method of forming an electron injecting/transporting layer from a material of a higher Ip (e.g., BAlq:Aluminum (III) bis(2-methyl-8-quinolinate)4-phenylphenolate) than the host material of the emitting layer and trapping holes in the emitting layer.
With this arrangement, hole blocking of the electron injecting/transporting layer can enhance probability of recombination of charges in the emitting layer, thereby providing phosphorescent emission with high efficiency.
However, on hole blocking, the holes concentrate on an interface between the emitting layer and the electron injecting/transporting layer. The concentration of the holes may promote degradation of the materials and reduce lifetime of the device. Accordingly, the electron injecting/transporting layer needs to be highly tolerant of the holes.    [Patent Document 1] JP-T-2004-506305    [non-Patent Document 1] Applied Physics letters Vol. 74 No. 3, pp 442-444    [non-Patent Document 2] Applied Physics letters Vol. 75 No. 1, pp 4-6